Field of the Invention
This application claims the benefit of Korean Patent Application No. 10-2013-0001321, filed on Jan. 4, 2013, which is hereby incorporated by reference as if fully set forth herein. Embodiments of the invention relate to an organic light emitting diode, and more particularly, to an organic light emitting diode micro-cavity structure and method of making. Although embodiments of the invention are suitable for a wide scope of applications, it is particularly suitable for a matrix structure having at least three sub-pixel regions.
Discussion of the Related Art
In general, an organic electroluminescent (EL) device, also known as organic light-emitting diode device or OLED device, can emit light at a predetermined color wavelength. An OLED device includes an anode for hole injection, a cathode for electron injection, and an organic emissive layer, sandwiched between the anode and cathode to enable hole and electron recombination that yields an emission of light. OLED devices can be stacked on top of one another so that one emissive layer emits one color and an other emissive layer emits another color such that the colors combine to form one or more colors. To construct a pixilated color display device such as a television, computer monitor, cell phone display, or digital camera display, individual OLED devices or stacked OLED devices can be arranged as a matrix array of pixels.
In order for each pixel to produce multiple colors, the pixels are divided into sub-pixel regions with each sub-pixel region having an OLED device for emitting a predetermined peak color wavelength or a plurality of color wavelengths. Generally, a color pixel display is made of one of two pixel types. The first pixel type is the RGB pixel type that has red, green and blue sub-pixel regions. The second pixel type is the RGBW pixel type that has red, green, blue and white sub-pixel regions. Red, green and blue sub-pixel regions emit predetermined peak color wavelengths of red, green and blue, respectively. A white sub-pixel region emits a plurality of color wavelengths. In addition to the RGB and RGBW pixel types, there are other display pixel types having sub-pixel regions with other predetermined peak color wavelengths, such as a RGCM pixel type having sub-pixel regions that emit with peak color wavelengths of red, green, cyan and magenta.
A matrix of pixels can be electrically driven using either a passive matrix or an active matrix driving scheme. In a passive matrix, the OLED sub-pixel regions are sandwiched between two sets of orthogonal electrodes arranged in rows and columns. In an active matrix configuration, each OLED device of a sub-pixel region is activated by switching element and driving element, such as transistors.
There are three basic approaches to using OLED device structures to produce a color display. The first approach uses different organic electroluminescent materials for the emissive layers in the sub-pixels of a pixel to emit different predetermined peak color wavelengths. The second approach uses a same organic electroluminescent material for each emissive layer along with different color filters in each sub-pixel region to emit different predetermined peak color wavelengths. The third approach uses different organic electroluminescent materials for the emissive layers along with along with different color filters in each sub-pixel region to emit different predetermined peak color wavelengths.
In the first approach of using different organic electroluminescent materials without color filters, each differently colored sub-pixel region can be constructed using different organic electroluminescent materials for each of the emissive layers of the OLED devices in the sub-pixel regions of the pixels. For example, a first organic electroluminescent material emits a peak red wavelength, a second organic electroluminescent material emits a peak green wavelength, and a third organic electroluminescent material emits a peak blue wavelength. These three different organic electroluminescent materials are each selectively positioned within their respective sub-pixel regions of the pixels by using one of shadow masks, thermal transfers from donor sheets, and ink jet printing.
In the second approach of using a same organic electroluminescent material for each emissive layer along with different color filters in each sub-pixel region to emit different predetermined peak color wavelengths to producing a color display, the same organic electroluminescent material in all of the different color sub-pixel regions can be either a continuous layer across all of the sub-pixel regions or individual layers position respectively in each sub-pixel region. The different color filters used to selectively convert the common color wavelength emitted from each OLED device to the different colors of the sub-pixel regions are positioned above the emissive layer in a top emission device and below the emissive layer in bottom emission device. In the case of using the same organic electroluminescent material in the emissive layer of all of the sub-pixel regions, the organic electroluminescent material is typically configured to produce a broad emission spectrum of light, also referred to as white emission or white light OLED.
In the third approach of using different organic electroluminescent materials for the emissive layers along with different color filters, each differently colored sub-pixel region can be constructed using different organic electroluminescent materials for each of the emissive layers of the OLED devices in the sub-pixel regions of the pixels. For example, a first organic electroluminescent material emits a peak red wavelength, a second organic electroluminescent material emits a peak green wavelength, a third organic electroluminescent material emits a peak blue wavelength and a fourth organic electroluminescent material emits a broad spectrum of wavelengths or white light. In this example, a red filter is associated with the first organic electroluminescent material that emits a peak red wavelength, a green filter is associated with the second organic electroluminescent material that emits a peak green wavelength, a blue filter is associated with the third organic electroluminescent material that emits a peak blue wavelength.
To improve the luminance output efficiency of the OLED devices in all three of the above approaches, the micro-cavity effect can be used. In a micro-cavity OLED device, the emissive layer structure is disposed between a reflector and a semi-transmissive reflector, which is at least semi-transparent to a desired wavelength. The reflector and semi-transmissive reflector form a Fabry-Perot micro-cavity that enhances the emission properties of the emissive layer structure disposed in the micro-cavity. More specifically, light emission near the wavelength corresponding to the resonance wavelength of the cavity is enhanced through the cathode while diminishing other wavelengths that do not correspond to the resonance wavelength. The use of a micro-cavity in an OLED device increases the light extraction efficiency or luminance output by configuring the depth or length of micro-cavity between a reflective electrode and a cathode of a sub-pixel region to have a resonance wavelength corresponding to the desired peak color wavelength for the sub-pixel region.
FIG. 1 is a view of three sub-pixel regions that each having a top emission OLED micro-cavity structure according to the prior art. As shown in FIG. 1, red sub-pixel region R, green sub-pixel region G and blue sub-pixel region B are formed on a substrate 101. The red sub-pixel region R includes a red emissive layer 151 positioned between a common cathode 160 and an anode 120R for the red sub-pixel region R. The green sub-pixel region G includes a green emissive layer 152 positioned between a common cathode 160 and an anode 120G for the green sub-pixel region G. The blue sub-pixel region B includes a blue emissive layer 153 positioned between a common cathode 160 and an anode 120B for the blue sub-pixel region B. The anode 120R for the red sub-pixel region R is on a cavity electrode 110R, which is on the substrate 101. The anode 120B for the blue sub-pixel region B is on a cavity electrode 110B, which is on the substrate 101. The anode 120G for the green sub-pixel region G is on a cavity electrode 110G, which is on the substrate 101. The red sub-pixel region R, green sub-pixel region G and blue sub-pixel region B are separated by banks 140.
The depth of a micro-cavity between a reflective electrode and a cathode of a sub-pixel region in the prior art is configured by controlling the thickness of the emissive layer. As shown in FIG. 1, the depth of the micro-cavity CDR in the red sub-pixel region R between the common cathode 160 and cavity electrode 110R for the red sub-pixel region R is larger than the depth of the micro-cavity CDG in the green sub-pixel region G between the common cathode 160 and the cavity electrode 110G for the green sub-pixel region G because the red emissive layer 151 of the red sub-pixel region R is thicker than the green emissive layer 152 of the green sub-pixel region G. The depth of the micro-cavity CDB in the blue sub-pixel region B between the common cathode 160 and the cavity electrode 110B for the blue sub-pixel region B is smaller than the depth of the micro-cavity CDG of the green sub-pixel region G between the common cathode 160 and the cavity electrode 110G for the green sub-pixel region G because the blue emissive layer 153 of the blue sub-pixel region B is thinner than the green emissive layer 152 of the green sub-pixel region G.
As shown in FIG. 1, the red color light RCL reflects back and forth between the common cathode 160 and the cavity electrode 110R across the entire depth of the micro-cavity CDR such that the luminance of the red color light RCL increases by constructive interference. The green color light GCL reflects back and forth between the common cathode 160 and the cavity electrode 110G across the entire depth of the micro-cavity GDR such that the luminance of the green color light GCL increases by constructive interference. The blue color light BCL reflects back and forth between the common cathode 160 and the cavity electrode 110B across the entire depth of the micro-cavity CDB such that the luminance of the blue color light BCL increases by constructive interference.
The effective micro-cavity depth is defined by optical distance, which is wavelength. The depth of the micro-cavity CDR for the red sub-pixel region R is configured to be deeper than the depth of the micro-cavity CDB for the blue sub-pixel region B because the wavelength of the red color light RCL is longer than the wavelength of blue color light BCL. The depth of the micro-cavity CDG for the green sub-pixel region G is configured to be deeper than the depth of the micro-cavity CDB for the blue sub-pixel region B because the wavelength of the green color light GCL is longer than the wavelength of blue color light BCL. The depth of the micro-cavity CDR for the red sub-pixel region R is configured to be deeper than the depth of the micro-cavity CDG for the red sub-pixel region G because the wavelength of the red color light RCL is longer than the wavelength of green color light GCL.
Controlling the thickness of the emissive layers is difficult. More specifically, the prior art structure requires a separate deposition process for each sub-pixel region because each emissive layer is typically a series of sub-layers whose thicknesses vary depending on the overall thickness of the emissive layer for that sub-pixel region. Further, the emissive layers can not be highly defined using the fine metal mask evaporation technique. Also, patterning of the transparent anode electrode in the prior art can result in left over residue that can result in a short circuit between adjacent pixel regions.